Biofuels produced from microalgae, at an estimated oil content of 40%, promise 10 to 100 times greater yield per land acre than other crops. Currently, a major hurdle for commercial-scale production of fuels from microalgae is recovering algal biomass from growth media with minimal cost and energy input. When cultivated in open raceway ponds, microalgae concentrations are very low—typically 0.1% (w/w). For fuel production, the challenge is therefore to harvest these dilute cultures efficiently and economically. Also, commercial-scale harvesting methods should also allow for recycling of water and unutilized water-soluble nutrients—important for environmental sustainability of microalgae production.
The current commercial methods for harvesting microalgae include centrifugation, filtration, and flocculation followed by settling or dissolved air flotation. Centrifugation can produce thick pastes with high solids concentration (˜20% (w/w)), and this method also allows for recovery of uncontaminated culture media for recycling. Recently, microalgae-specific low-speed centrifuges have been developed by Evodos™ that require less energy (8 kWh/m3 of media processed) than more conventional centrifuges used in the biotechnology and fermentation industries. Cell lysis can also be prevented in the Evodos™ models. Nonetheless, the capital costs associated with centrifugation are unappealing.
Cross flow filtration is another method that has primary use in biotechnology/fermentation, but could also be applied for separation of microalgae. With this technique, reasonably high biomass concentrations can be achieved (8% (w/w)), and since no chemicals are added in this process, the recovered permeate can be recycled. When solids concentrations are low, such as in microalgae cultures from open raceway ponds, the energy use is also low due to small cake resistance (approximately 2 kWh/m3). However, the primary concern with cross flow filtration is the cost associated with membrane replacement and membrane cleaning. Due to the presence of exo-polysaccharides in many algal cells, irreversible fouling of membranes is unavoidable in commercially available 0.22 μm cross flow filtration modules, necessitating frequent membrane replacements. Fouling issues could be avoided, at least in part, by use of membranes with larger pore sizes (3-5 μm). However, low-cost large-pore membrane modules are not commercially available and may be challenging to manufacture since conventional polymer membrane supports have sub-micron pore sizes.
Flocculation followed by settling or dissolved air flotation, adapted from wastewater treatment, can also be used for harvesting microalgae. With this technique too, reasonably high slurry concentrations can be achieved (5-10% (w/w)) with energy consumption varying between 10-20 kWh/m3. However, addition of flocculants precludes the recycling of growth media for microalgae re-cultivation. Also, the carryover of flocculants with the harvested biomass could negatively impact downstream conversion processes and fuel quality.
Historical observations have shown that some microalgae species are also able to “autoflocculate” under alkaline conditions. Investigations to elucidate the mechanism for this occasional spontaneous settling of microalgal cells show that the process might actually be a result of chemical flocculation due to precipitation of calcium and magnesium hydroxides (from Ca and Mg salts in the growth media) when solution pH is high. Further, this process results in only a small increase in concentration of cells (up to 0.1 to 0.5%). Secondary harvesting methods are therefore still needed to increase concentrations to levels compatible with conversion processes (˜10% (w/w)). In any case, auto flocculation appears to be species-specific and this method is likely applicable only to a few select strains.
It is clear that while harvesting methods from other industries may be applied to microalgae cultures, they are not particularly well-suited. Cross flow filtration and centrifugation are best suited for biotechnology applications with high cell concentrations of fermentation cultures and a high value of the products. Flocculation works well in wastewater treatment since recyclability of water and quality of recovered biomass are not of primary concern. Clearly, transformative alternatives for dewatering dilute microalgae slurries are still needed.